1. Field of the Invention
The present invention relates to a nitride semiconductor wafer, a nitride semiconductor chip, and a method of manufacture of a nitride semiconductor chip.
2. Description of Related Art
Nitride semiconductors as exemplified by GaN, AlN, InN, and their mixed crystals are characterized by having wider band gaps Eg than AlGaInAs- and AlGaInP-based semiconductors and in addition being direct band gap materials. For these reasons, nitride semiconductors have been receiving attention as materials for building semiconductor light-emitting chips such as semiconductor laser chips emitting light in wavelength regions from ultraviolet to green and light-emitting diode chips covering wide emission wavelength ranges from ultraviolet to red, and are expected to find wide application in projectors and full-color displays, and further in environmental, medical, and other fields.
On the other hand, in recent years, many research institutions have been making vigorous attempts to realize semiconductor light-emitting chips emitting light in a green region (green semiconductor lasers) by making longer the emission wavelengths of semiconductor light-emitting chips using nitride semiconductors.
Generally, in a semiconductor light-emitting chip using a nitride semiconductor, a substrate (nitride semiconductor substrate) of GaN, which has a hexagonal crystal system, is used, and its c plane (the (0001) plane) is used as the principal growth plane. By stacking nitride semiconductor layers including an active layer on the c plane, a nitride semiconductor light-emitting chip is formed. Generally, in a case where a nitride semiconductor light-emitting chip is formed by use of a nitride semiconductor substrate, an active layer containing In is used, and by increasing the In composition ratio, a longer emission wavelength is sought.
Inconveniently, however, the c plane of a GaN substrate is a polar plane having polarity in the c-axis direction, and therefore stacking nitride semiconductor layers including an active layer on the c plane causes spontaneous polarization in the active layer. Also inconveniently, when nitride semiconductor layers including an active layer are stacked on the c plane, as the In composition ratio increases, lattice distortion increases, inducing in the active layer a strong internal electric field due to piezoelectric polarization. The internal electric field reduces the overlap between the wave functions of electrons and holes, and thus diminishes the rate of radiative recombination. Accordingly, increasing the In composition ratio in an attempt to realize light emission in a green region suffers from the problem that, as the emission wavelength is lengthened, luminous efficacy significantly lowers.
To avoid the effects of spontaneous polarization and piezoelectric polarization, therefore, there are nowadays proposed nitride semiconductor light-emitting chips having nitride semiconductor layers stacked not on the c plane as commonly practiced but on the m plane (the {1-100} plane), which is a non-polar plane. Such nitride semiconductor light-emitting chips are disclosed, for example, in JP-A-2008-91488.
The nitride semiconductor light-emitting chip (light-emitting diode chip) disclosed in JP-A-2008-91488 mentioned above is provided with a GaN substrate of which the m plane, which is a non-polar plane, is used as the principal growth plane, and on this principal growth plane (the m plane), nitride semiconductor layers including an active layer are stacked. The m plane is a crystal plane perpendicular to the c plane, and therefore stacking nitride semiconductor layers including an active layer on the m plane causes the c axis, which is an axis of polarization, to lie on the plane of the active layer. Thus, the effects of spontaneous polarization and piezoelectric polarization are avoided, and the lowering of luminous efficacy is suppressed. Incidentally, in the nitride semiconductor light-emitting chip (light-emitting diode chip) disclosed in JP-A-2008-91488, from the viewpoint of suppressing deterioration of surface morphology, the m plane of the GaN substrate is so adjusted that its off-angle (alignment error) is not more than ±1 degree.
As described above, by use of a nitride semiconductor substrate having the m plane as the principal growth plane, it is possible to obtain a nitride semiconductor light-emitting chip in which the lowering of luminous efficacy due to spontaneous polarization and piezoelectric polarization is suppressed.
Inconveniently, however, through measurement of luminous efficacy (of light emission resulting from current injection, that is, electroluminescence, abbreviated to EL) with regard to nitride semiconductor light-emitting chips using a nitride semiconductor substrate having the m plane as the principal growth plane, it was confirmed that, as the In composition ratio in the active layer increased, the luminous efficacy sharply lowered. Through intensive studies in search of the cause of the phenomenon, the inventors of the present invention have found out that the lowering of luminous efficacy is caused by the EL emission pattern (the light distribution across the plane as observed when light is emitted by current injection) becoming bright-spotted. That is, the inventors have found out that, as the In composition ratio in the active layer increases, the EL emission pattern of nitride semiconductor light-emitting chips become bright-spotted.
Specifically, when nitride semiconductor light-emitting chips (light-emitting diode chips) using a nitride semiconductor substrate having the m plane as the principal growth plane were fabricated and were made to emit light by current injection, a bright-spotted EL emission pattern as shown in FIG. 47 was observed. What conditions cause this phenomenon have not conventionally been known at all. Through close studies in search for the cause, it has been found out that, as the In composition ratio in the active layer increases, the EL emission pattern becomes increasingly bright-spotted. Such a bright-spotted EL emission pattern becomes more prominent as the In composition ratio in the active layer increases, and a tendency has been observed that a bright-spotted EL emission pattern is especially prominent starting around a green region (with the In composition ratio in the active layer (well layer) 0.15 or more). As the In content increases further, the number of light-emitting bright spots (the area of light emission) decreases. Thus, a strong correlation is observed between the bright-spotted EL emission pattern and the In composition ratio, and it has therefore been found out that the phenomenon of the EL emission pattern becoming bright-spotted causes the lowering of luminous efficacy that occurs with increased In composition ratios in the active layer. Used as the nitride semiconductor substrate in the studies was a just substrate with an off-angle of 0 degrees (that is, a substrate with no off-angle).
The bright-spotted EL emission pattern described above is a phenomenon prominent in nitride semiconductor light-emitting chips using a nitride semiconductor substrate having a non-polar plane, in particular the m plane, as the principal growth plane.
As discussed above, it has been found out that, in nitride semiconductor light-emitting chips using a nitride semiconductor substrate having the m plane as the principal growth plane, as distinct from nitride semiconductor light-emitting chips using the c plane, the lowering of luminous efficacy due to spontaneous polarization and piezoelectric polarization is suppressed, but there is the problem of lower luminous efficacy due to a bright-spotted EL emission pattern. In nitride semiconductor light-emitting chips using the m plane, such a bright-spotted EL emission pattern poses a great problem because it hampers the lengthening of the emission wavelength. In particular, in semiconductor laser chips, low luminous efficacy is a serious problem because it leads to low gain.
On the other hand, in nitride semiconductor light-emitting chips (nitride semiconductor chips) such as nitride semiconductor laser chips, when a nitride semiconductor layer is grown on the m plane of a nitride semiconductor substrate, a difference in lattice constant, thermal expansion coefficient, etc. between the nitride semiconductor substrate and the nitride semiconductor layer may produce strain in the nitride semiconductor layer, and the strain may cause a crack to develop in the nitride semiconductor layer. Development of a crack in the nitride semiconductor layer reduces the number of acceptable chips obtained from a single wafer, and thus leads to the problem of low yields. Development of a crack also diminishes reliability and degrades chip characteristics such as the emission lifetime. It is therefore very important, from the perspective of chip production, to suppress development of cracks.
In particular, in the fabrication of semiconductor light-emitting chips emitting light in an ultraviolet region or those emitting light in a green region (for example, green semiconductor lasers), for effective light confinement, a semiconductor layer whose lattice constant greatly differs from that of a substrate may be formed on the substrate. In this case, cracks develop very easily, leading to the problem that it is extremely difficult to enhance chip characteristics and increase yields.